1. Field of the Invention
The present invention relates to a thin film dielectric device having a dielectric thin film composed of a perovskite oxide.
2. Description of the Related Art
In recent years, materials with high permittivity and ferroelectricity of thin film dielectric devices (thin film capacitors) that are disposed on large storage DRAMs and non-volatile ferroelectricity memories (FRAMs) and device structures thereof have been intensively studied. For example, the dielectric constants of perovskite oxides such as SrTiO.sub.3 (hereinafter referred to as STO) and Ba.sub.1-x Sr.sub.x TiO.sub.3 (hereinafter referred to as BSTO) are much larger than the dielectric constants of conventional materials such as SiO.sub.2, Si.sub.3 N.sub.4, and Ta.sub.2 O.sub.5. Thus, even if memory cells are finely structured, a sufficient storage capacity of electric charges can be obtained. In addition, a desired storage capacitance can be accomplished without need to use a complicated capacitor structure.
In a thin film dielectric device having a dielectric thin film composed of a perovskite oxide, it is considered that a noble metal such as Pt or Ru, an oxide of such as Ru, or a laminate film thereof to be a lower electrode. Among them, Ru has an excellent machinability and can be finely patterned by reactive ion etching (RIE) method or the like. Thus, it was considered that Ru is an excellent material of a capacitor electrode.
However, in a thin film dielectric device of which Ru or an oxide thereof is used for a lower electrode, a large amount of interface levels will take place due to ion defeats resulting from the mismatching of the interface an electrode and a dielectric thin film. The interface levels causes a leak current to increase and a dielectric breakdown resistance to decrease.
On the other hand, it is considered to use a conductive perovskite oxide having the same crystal structure as the above-described STO and BSTO for the material of an electrode, in particular, for the material of a lower electrode. When a conductive perovskite oxide is used for a lower electrode and it is epitaxially grown from the lower electrode to a dielectric thin film composed of the perovskite oxide or to an upper electrode, the lower electrode and the dielectric thin film are highly matched at the interface thereof. In addition, the dielectric thin film and the upper electrode are highly matched at the interface thereof. It is expected that such an epitaxially grown film suppresses the leak current due to a large amount of interface levels from increasing and the dielectric breakdown resistance from degradation.
To actually fabricate a thin film dielectric device that is epitaxially grown, a single crystal substrate composed of MgO or SrTiO.sub.3 should be used for a substrate. However, in a real LSI circuit or the like, the lower electrode should be formed on a plug composed of poly-silicon or tungsten (Si plug or W plug). It is very difficult to epitaxially grow the lower electrode composed of a conductive perovskite oxide on a plug composed of poly-silicon in a good condition. Thus, the electrode and the dielectric thin film cannot be highly matched at the interface thereof.
On the other hand, it is considered to use a polycrystalline film composed of a conductive perovskite oxide for a lower electrode. For example, a structure of which a polycrystalline film composed of a conductive perovskite oxide such as SuRuO.sub.3 is formed on a plug composed of poly-silicon or the like through a barrier metal layer composed of such as TiN as a lower electrode and then a dielectric thin film composed of for example BSTO is formed on the polycrystalline film has been considered. However, in such a structure, the morphology degrades at the interface of the lower electrode and the dielectric thin film and at the interface of the dielectric thin film and the upper electrode. The degradation of the morphology at the interface causes the leak current to increase or a capacitor to short-circuit.
As described above, in a conventional thin film dielectric device having a dielectric thin film composed of perovskite oxide, when a noble metal such as Ru or an oxide thereof is used for a lower electrode, since the dielectric thin film and the lower electrode are not matched at the interface thereof, the leak current increases or the dielectric breakdown resistance decreases.
When the conductive perovskite oxide is used for the lower electrode and epitaxially grown, the substrate thereof is restricted. Thus, an epitaxial film of the conductive perovskite oxide cannot be used for a real LSI circuit or the like due to restrictions on process and device structure. When a polycrystalline film of the conductive perovskite oxide is used for the lower electrode, the morphology degrades at the interface thereof. The degradation of the morphology at the interface causes the leak current to increase or a capacitor to short-circuit.
Thus, in the thin film dielectric device having the dielectric thin film composed of the perovskite oxide, the lattice constant of the electrode and the lattice constant of the dielectric thin film should be highly matched. In addition, when the conductive perovskite oxide is used for the electrode, the flatness of the surface of the polycrystalline film should be improved. Thus, the problems of which the leak current increases and the dielectric breakdown resistance decreases should be solved. In addition, such a thin film dielectric device should be applied to a real LSI or the like.